1. Technical Field
The present invention relates generally to color thermal printing, and, more particularly, to an apparatus and method for positioning a dye donor web relative to the print head with high precision for minimal waste.
2. Background Art
In a color thermal printing process, the finished print is made by successively transferring yellow, magenta, and cyan dyes from respective dye patches of a dye donor web onto a dye receiver medium. In one type of color thermal printer, such as disclosed in commonly assigned U.S. Pat. No. 4,710,781 which issued to Stanley W. Stephenson on Dec. 1, 1987, a dye donor web contains a repeating series of frames of different colored, heat transferable dyes. The dye donor web is disposed between a receiver medium, such as coated paper and a print head formed of a plurality of individual resistive heating elements. When a particular heating element is energized, its heat causes dye from the donor web to transfer to the receiver medium.
Conventionally, the yellow frame is first positioned under the print head with the receiver medium. As they are advanced, the heating elements are selectively energized to form a yellow dye image on the receiver medium. Next, the magenta frame and the receiver medium are moved under the print head. Both the receiver medium and the magenta frame are moved as the heating elements are selectively energized, whereby a magenta dye image is formed, superimposed upon the yellow image. This process is repeated for the remaining frames of the series, such that a multi-color image is formed on the receiver medium.
During the color printing process, it is necessary to have the dye donor web properly positioned relative to the dye receiver medium to ensure full coverage of the image area by successive color frames. Since the donor web has a repeating series of different colored dye frames, it is necessary to identify the leading edge of each different frame of each series. One way to do this is to provide index marks on the donor; one type of mark for the yellow frame (the first frame of each series) and a different type of mark for the other color frames.
Another way to identify the frames is disclosed in above-identified Stephenson patent, wherein color discriminating optical sensors are located directly in the donor web path just past the print line of the thermal print head in the direction of travel of the donor web. These sensors detect the presence of different colored patches on the donor as they advance. A beam of red light and a beam of yellow light are passed through the donor web near to the print head. Respective photodetectors measure the intensity of the beams passing through the web. The particular dye frame can be identified by analyzing the two light beams because of transmission differences of the color dyes. When the sensors detect a new color frame during the printing cycle, the donor web advance is stopped.
In the system of the Stephenson patent, it is desirable to position the sensors as close as possible to the print line of the thermal print head because donor web that occupies that distance after positioning is not used in printing, and is therefore wasted. Unfortunately, the physical configuration of the print head and surrounding mechanisms limit the minimum distance that can be achieved. This, in turn, limits the minimum size of the color frames. Among the consequences of having unused donor are: a higher cost of materials for making prints, reduced donor web capacity in the printer, and a greater volume of material requiring environmentally safe disposal after use.
Accordingly, it will be appreciated that it would be highly desirable to have a thermal printer in which the amount of dye donor web that is wasted by inability to locate the sensor at the print line of the thermal print head is minimized. In commercially-available printers of the type described in the Stephenson patent, the problem of donor web waste as set forth above is partially overcome by rewinding the donor after the leading edge of a color frame is sensed. A motor on the donor web supply spool is programmed to rotate the supply spool through a predetermined arc length in the reverse-feed direction to draw the web backwards; returning the leading edge of the color patch toward the print line of the thermal print head. However, since the distance of web travel is not only a function of the amount of supply spool rotation, but also a function of supply roll diameter, the amount of rotation of the supply roll must be determined for a full supply roll. Thus, operation with anything other than a full supply roll still results in additional dye donor web waste.